Studies performed in the Tumor Biological Research Unit of the Hadassah-Hebrew University Hospital-Israel (Isr. Med. Assoc. J. 2000, 2, 37-45; J. Med. Chem. 2000, 43, 2591-600; Invasion Metastasis 1994-95, 14, 290-302; Exp. Cell Res. 1992, 201, 208-15) focus on the involvement of heparin-binding growth factors, heparan sulphate and heparan sulphate-degrading enzymes (heparanase) in tumor angiogenesis and metastasis. These studies have been applied to screening and to the identification of heparin derivatives and heparin/heparan sulphate mimetics with potent heparanase inhibiting activity (Nature Med. 1999, 5, 735-6; Science, 1999, 285, 33-4].
Tumor cells release the enzyme heparanase, an endo-□-D-glucuronidase which degrades the polysaccharide chain of heparan sulphate proteoglycans on cell surfaces and in the extracellular matrix.
Involvement in tumor angiogenesis of heparanase has been correlated with the ability to release bFGF (FGF-2) and other growth factors from its storage within the ECM (extracellular matrix). These growth factors provide a mechanism for induction of neovascularization in normal and pathological situations.
Heparanase may thus facilitate not only tumor cell invasion and metastasis but also tumor angiogenesis, both critical steps in tumor progression.
Specific inhibitors of the heparanase enzyme prevent release and activation of growth factors stored by heparan sulphate as well as disruption of the ECM, and are regarded as a very promising approach to develop anticancer drugs.
So, one of possible therapeutic approaches for an antiangiogenic drug is the development of a potent and selective heparanase inhibitor.
For a discussion of angiogenesis, reference may be made to WO 01/55221, in the name of the present applicant.
Another important involvement of heparanase is both inflammation and autoimmunity. In fact, heparanase activity correlates also with the ability of activated cells of the immune system to leave the circulation and elicit both inflammatory and autoimmune responses. Interaction of platelets, granulocytes, T and B lymphocytes, macrophages and mast cells with the subendothelial ECM is associated with degradation of heparan sulphate by heparanase activity. The enzyme is released from intracellular compartments (i.e. lysosomes, specific granules) in response to various activation signals, suggesting its regulated involvement and presence in inflammatory sites and autoimmune lesions. Treatment of experimental animals with heparanase inhibitors (i.e., non-anticoagulant species of low molecular weight heparin—LMWH) markedly reduced the incidence of experimental autoimmune encephalomyelitis (EAE), adjuvant arthritis and graft rejection in experimental animals, indicating that heparanase inhibitors may be applied to inhibit autoimmune and inflammatory disease.
Heparin
Heparin is a heterogeneous mixture of naturally occurring polysaccharides of various lengths and various degrees of sulphation which possesses anticoagulant activity and is secreted by the connective tissue mast cells present in the liver (from which it was first isolated), in the muscles, lungs, thymus and spleen.
In addition to the regular sequence, a sequence corresponding to the active site for antithrombin activity has been identified in heparin.
The antitumor and antimetastatic activity of heparin and its derivatives is due to its ability to inhibit heparanase, to block growth factors and to regulate angiogenesis.
Heparan Sulphates (HS)
Heparan sulphates (HS) are ubiquitous protein ligands. The proteins bind to the HS chains for a variety of actions from simple immobilisation or protection against the proteolytic degradation action to specific modulations of biological activities, such as angiogenesis.
The carbohydrate skeleton, in both heparin and the heparan sulphates (HS), consists in an alternation of D-glucosamine (GlcN) and hexuronic acids (GlCA or IdoA).
In heparin, the GlcN residues are mainly N-sulphated, whereas in HS they are both N-sulphated and N-acetylated, with a small amount of unsubstituted NH2 groups.
HS is also on average less O-sulphated than heparin.
The use of heparin in the treatment of angiogenesis disorders, such as tumours, particularly metastases, is substantially limited by the anticoagulant activity of heparin.
Chemical modifications have been made to heparin so as to reduce its anticoagulant capacity, at the same time preserving its antitumor properties.
The opening of a unit of glucuronic acid in the antithrombin site reduces the affinity of heparin for antithrombin: in this way, heparins can be used with reduced anticoagulant effects, but still retaining antiangiogenic properties.
Heparanases
Heparanases are enzymes belonging to a family of endoglycosidases (an endo-□-D-glucuronidase) that hydrolyse the internal glycoside bonds of the chains of heparan sulphates (HS) and heparin.
These endoglycosidases are involved in the proliferation of tumour cells, in metastases and in the neovascularisation of tumours. These enzymes are biological targets for antiangiogenic activity. In the scientific literature there are a large number of structure/activity correlation studies (see, for example, Lapierre F. et al., Glycobiology, vol. 6, (3), 355-366, 1996). Though many aspects have still to be clarified, studies have been reported regarding the inhibition of heparanases by heparin and its derivatives, using specific tests which have led to the emergence of considerations of a structural type which may serve as guides for obtaining new, more selective derivatives.
In the above-mentioned work of Lapierre et al., heparin derivatives are described as obtained by 2-O desulphation or by “glycol split” (oxidation with periodate and subsequent reduction with sodium borohydride). These derivatives, defined here as “2-O-desulphated heparin” and “RO-heparin”, respectively, have partly maintained the antiangiogenic activity of heparin as assessed by means of the CAM test in the presence of corticosteroids (ibid. page 360).
N-acyl heparin derivatives, which are closer mimics of heparan sulphate than heparin, have been reported to inhibit heparanase only somewhat less than N-sulphate derivatives. (Irimira T., Biochemistry 1986, 25, 5322-5328; Ishai-Michaeli R., et al, Biochemistry 1992, 31, 2080-2088).
Heparins and FGF
FGFs regulate multiple physiological processes such as cell growth and differentiation, but also functions involved in pathological processes such as tumour angiogenesis.
FGFs are growth factors (a family of more than 10 polypeptides, of which the acid (FGF-1) and basic FGFs (FGF-2) are the ones which have been most studied, which require a polysaccharide cofactor, heparin or HS, to bind to the FGF receptor (FGFR) and activate it.
Though the precise mechanism whereby heparin and HS activate FGFs is unknown, it is known, however, that heparin/FGF/FGFR form a “trimolecular” or “ternary” complex.
One mechanism postulated is that heparin and HS induce so-called sandwich dimerisation of FGF, and the latter, thus dimerised, forms a stable complex with FGFR.
Antimetastatic Activity of Heparin derivatives
The ability of a primary tumour to generate metastatic cells is perhaps the main problem facing anticancer therapy.
Heparin derivatives with a substantial ability to block heparanase seem to be equally capable of inhibiting angiogenesis both in primary tumours and in metastases.
In addition, the inhibition of heparanase reduces the migration ability of tumour cells from the primary tumour to other organs.
The antimetastatic activity in animal models has been found to correlate with the heparanase-inhibiting ability of heparin and heparin derivatives (Bitan M. et al, Isr. J. Med. Sci. 1995, 31, 106-108) as well as other sulphated polysaccharides (Miao, H. Q. et al, Int. J. Cancer 1999, 83, 424-431, and references therein). Studies on the molecular-weight dependence of the antimetastatic activity indicated that also very low-MW heparins (Sciumbata, T., et al, Invasion Metastasis 1996, 16, 132-143) and oligosaccharide polysulphates (Parish, C. R., et al, Cancer Res. 1999, 59, 3433-3441) retain significant antimetastatic activity. Although in general removal of N-sulphate groups (N-desulphation) decreases the antimetastatic potential of heparins, this activity is partially restored upon N-acylation (N-acetylation, N-hexanoylation (Bitan M., 1995), and N-succinylation (Sciumbata, T., 1996) of resulting free NH2 groups. The antimetastatic activity of heparins was found to be inversely correlated to their degrees of O-sulphation. (Bitan M., 1995). However, selective 2-O-desulphation of iduronic acid residues did not involve a strong reduction of the antimetastatic activity of heparin (Lapierre, F., Glycobiology 1996, 6, 355-366).
In general, both the heparanase-inhibiting and the antimetastatic activity of heparins and other sulphated polysaccharides decrease with decreasing molecular weight and degree of sulphation (Bitan M., 1995; Parish, C. R., 1999). However, these activities also depend on the carbohydrate backbone of the polysaccharide (type of residues and position of glycosydic linkages) (Parish, C. R., 1999). Since the tridimensional structure of the active site of heparanase is not yet known, it is difficult to predict which polysaccharide backbones and sulphation patterns most effectively inhibit the enzyme.
On the basis of the present knowledge, the structural requirements of heparin-like molecules that favour the angiogenesis-inhibiting action can be grouped in two categories on the basis of the target one intends to block:
a) inhibition of heparanase: although this enzyme recognizes and cleaves heparin and HS sequences of at least eight monosaccharide units containing N-acyl-glucosamine-glucuronic acid (or N-sulphated glucosamine residues see, for example, D. Sandback-Pikas et al. J. Biol. Chem., 273, 18777-18780 (1998) and references cited), its inhibition can be efficiently accomplished by heparin fragments longer than tetradecasaccharide (Bitan M., 1995) or by extensively sulphated, shorter oligosaccharides, such as maltohexaose sulphate (MHS) and phosphomannopentaose sulphate (PI-88) (Parish, C. R., 1999). However, both long heparin fragments and heavily sulphated oligosaccharides are anticoagulant, a property that should be avoided for potential antimetastatic drugs;
b) inhibition of angiogenic growth factors (fibroblast type: FGF-1 and FGF-2; vascular endothelium type: VEGF; vascular permeability type: VPF): to this end the heparin-like compounds preferably have sequences at least five monosaccharide units long, containing 2-sulphated iduronic acid and glucosamine N,6-sulphated (see, for example, M. Maccarana et al. J. Biol. Chem., 268, 23989-23905 (1993)).
The literature discloses small peptides (5-13 amino acids) with antiangiogenic activity (U.S. Pat. No. 5,399,667 of the University of Washington) which act by binding to a thrombospondin receptor, or longer peptides (50 amino acids approx.).
Modified platelet factors are known—(EP 0 589 719, Lilly), capable of inhibiting endothelial proliferation, with IC50=7 nM.
Oligosaccharide fragments with antiangiogenic activity have also been amply described: it has been found, in fact, that by varying the carbohydrate sequence the interaction selectivity can be increased.
In addition, heparin can be used as a vehicle for substances which are themselves antiangiogenic, such as some steroids, exploiting the affinity of heparin for vascular endothelial cells; see, for example, WO 93/18793 of the University of Texas and Imperial Cancer Research Technology, where heparins are claimed with acid-labile linkers, such as adipic acid hydrazine, bound to cortisol. The antiangiogenic effect of the conjugated molecules is greater than that of the unconjugated molecules, even when administered simultaneously.
In Biochim Biophys. Acta (1996), 1310, 86-96, heparins bound to steroids (e.g. cortisol) are described with a hydrazone group in C-20 that present greater antiangiogenic activity than the two unconconjugated molecules.
EP 0 246 654 by Daiichi Sc. describes sulphated polysaccharides with antiangiogenic activity with Phase II studies. EP 0 394 971 by Pharmacia & Upjohn—Harvard Coll. describes hexa-saccharides—heparin fragments—with low sulphation, capable of inhibiting the growth of endothelial cells and angiogenesis stimulated by FGF-1. EP 0 618 234 by Alfa Wasserman describes a method for preparing semisynthetic glycosaminoglycans with a heparin or heparan structure bearing a nucleophilic group. WO 95/05182 by Glycomed describes various sulphated oligosaccharides with anticoagulant, antiangiogenic and anti-inflammatory activity. U.S. Pat. No. 5,808,021 by Glycomed describes a method for preparing substantially non-depolymerised 2-O, 3-O desulphated heparin with a desulphation percentage in positions 2- of the iduronic acid (I, 2-O) and in position 3 of the glucosamine unit (A, 3-O) ranging from approximately 99 to approximately 75% of the original percentage. This method envisages desulphation conducted in the presence of a cation of a bivalent metal, exemplified by calcium or copper, followed by lyophilisation of the product obtained. The desulphated heparins have antiangiogenic activity. EP 0 251 134, Yeda Res & Dev Co Ltd et al, discloses the use of subcoagulant dosages of heparin or its derivatives for preventing allograft rejection and treating autoimmune diseases. The activity of heparin is given by inhibition of heparanase. WO 88/05301, Univ. Australian Nat., discloses antimetastatic and/or antiinflammatory compositions containing a sulphated polysaccharide, which is heparanase inhibitor. Heparin, fucoidan, pentosan sulphate, dextran sulphate are provided. WO 92/01003, Univ. Texas System, discloses the use of a heparin derivative, which is devoid of anticoagulation activity, as heparanase inhibitor. These derivatives have sulphamino or O-sulphate groups, M.W. 1000-15000 and each terminal monomeric unit is a monomeric repeating unit with a terminal O atom bound to a blocking group. WO 94/14851 and WO 96/06867, Glycomed, provide 2-O, 3-O-de-sulphated mucosal heparin, or fragments thereof, being at least 96.7% de-sulphated at the 2-O position and ate least 75% desulphated at the 3-O position useful as non-anticoagulant heparanase inhibitors. WO 95/09637 and WO 96/09828, Glycomed, discloses highly sulphated maltooligosaccharide compounds with heparin like properties. WO 95/30424, Glycomed, provides 6-O-desulphated heparin or fragments thereof with heparanase inhibiting activity. WO 96/33726, Univ. Australian Nat., discloses sulphated oligosaccharides as heparan mimetics having heparanase inhibiting activity. WO 01/35967, Knoll A G, provides a method for treating cardiac insufficiency and related conditions by administering an heparanase inhibitor, among which, heparin which has partly reduced COOH groups, or is at least partly N-desulphated and N-acetylated or is at least partly N,O-desulphated and N-resulphated or is O-acetylated is mentioned.
The aim of the invention described herein is to find optimal glycosaminoglycan structures for generating antiangiogenic activity based on heparanase inhibition and/or FGF growth factor inhibition mechanisms. An additional aim of the invention described herein is to provide a medicament with antiangiogenic activity which is essentially devoid of the typical side effects of heparin derivatives, such as, for example, anticoagulant activity.
WO 01/55221, in the name of the applicant, discloses glycosaminoglycans, particularly a desulphated heparin, with a desulphation degree not greater than 60% of the total uronic units. These derivatives are provided with antiangiogenic activity and are devoid of anticoagulant activity. Said compounds exert their antiangiogenic activity based on the inhibition of FGF. No activity was foreseen for inhibition of heparanase.
In quite general terms, WO 01/55221 also provides a modified heparin, containing glycosamine residues with different degrees of N-desulphation and optional subsequent total or partial acetylation. The general teaching of said reference does not explicitly describe the N-desulphation and optional subsequent total or partial acetylation steps.